1. Field of the Invention
The invention relates to a process for preparing silanes from hydrogen chloride gas and silicon metal, disilanes, and oligosilanes as reactants.
2. Background Art
In the preparation of chlorosilanes, silicon is reacted with hydrogen chloride gas, if appropriate in the presence of catalysts, resulting, depending on the reaction conditions, in the formation of trichlorosilane or tetrachlorosilane as the main product. The reaction temperature selected depends greatly on the desired target product (HSiCl3 or SiCl4) and is normally from 230xc2x0 C. to several hundred xc2x0 C. Trichlorosilane is used, for example, for the preparation of high-purity silicon or the preparation of (organo)functional silanes, while tetrachlorosilane is used, for example, in the preparation of pyrogenic silica. The relevant technology is described in Catalyzed Direct Reactions of Silicon, edited by K. L. Lewis and D. G. Rethwisch, Ed.s Elsevier 1993; xe2x80x9cCommercial Production of Silanes by Direct Synthesis,xe2x80x9d pages 1-66 and xe2x80x9cDirect Synthesis of Chlorosilanes and Silane,xe2x80x9d pages 441-457.
In the direct synthesis of methylchlorosilanes by the Muller-Rochow method (direct synthesis), silicon is reacted with methyl chloride in the presence of catalysts and promoters to form dimethyldichlorosilane, with higher-boiling methylchlorodisilanes and methylchlorooligosilanes and the corresponding siloxanes and carbosilanes also being formed as by-products. Lewis, op. cit., pp. 1-66. U.S. Pat. No. 5,877,337 describes a process in which solids-containing residues from the direct synthesis of organochlorosilanes are worked up at low pressures and the organosilicon components are converted into useful silanes. This objective is achieved by carrying out thermal cracking of the residues from the direct synthesis in the presence of hydrogen chloride in a tube reactor containing rotating internals at temperatures of 300-800xc2x0 C. The rotary motion of the internals shears off deposits on the reactor walls caused by carbon formation or solid components. This is said to prevent blocking of the reactor.
In these processes, hydrogen chloride gas is required and the reactions are carried out at relatively high temperatures. The hydrogen chloride gas is taken from appropriate storage tanks and may have to be preheated. In Lewis, op. cit., for example, it is stated on page 6 that liquid anhydrous hydrogen chloride which is used in the direct synthesis of chlorosilanes is typically transported to production sites in cooled tank cars. HCl is vaporized, heated to about 200xc2x0 C. and subsequently introduced into the reactor.
The preparation of HCl from the elements is known. H2 and Cl2 are introduced separately into a combustion chamber, mixed and reacted in a burner flame. The reaction is strongly exothermic, so that flame temperatures of above 2000xc2x0 C. are possible. The hydrogen chloride gas is subsequently cooled by means of appropriate cooling apparatuses.
The present invention avoids all or a portion of the energy penalty required to heat hydrogen chloride gas prior to the commercial preparation of silanes by the in situ generation of hydrogen chloride by combustion of hydrogen and chlorine. Numerous advantages other than thermal efficiency surprisingly accrue from this process.